Deliverable

• Identification of zones of potential weakness under the external forcing of climate change and eutrophication for future political and hazard prevention measures

Gregor Rehder, B.B. Jørgensen.

This deliverable provides an integrated view of different efforts within the BALTIC GAS project with the scope to assess the potential for future increase of gas ebullition, and changes in the spatial extent of gassy sediments as background for the evaluation of Greenhouse Gas emissions by environmental authorities as well as background for future offshore-activities and construction in or near gas-rich sedimentary basins of the Baltic Sea.

Gas seepage is known or has been identified during the BALTIC GAS Project from the Kattegat and a limited number of locations in the Baltic Sea Shallow gas areas have been identified and mapped within the depositional centers of organic-rich Holocene mud deposits. The effect of eutrophication and regional climate change and with this, seafloor temperature, have been addressed by a transient reaction-transport model. The physical properties of gas-bearing sediments have been systematically investigated, with special emphasis to assess the seafloor’s capacity to withstand the buoyancy force of embedded free gas and its potential for slowly fracturing its way up through the mud.

This deliverable provides an integrated view of these different efforts within the BALTIC GAS project with the scope to assess the potential for future increase of gas ebullition, and changes in the spatial extent of gassy sediments as background for the evaluation of Greenhouse Gas emissions by environmental authorities and for future offshore-activities and construction in or near gas-rich sedimentary basins of the Baltic Sea.

Synopsis

• Only few locations with ebullition of free gas have been reported and observed in the Baltic Sea, despite the fact that in regions where young, organic-rich Holocene mud is accumulating, large areas with occurrence of free gas in the upper sediment layer have been mapped. Low permeability and a lack of overpressure generally prevent advective gas migration.
• Though the combined effects of enhanced primary production and projected climatic changes in the Baltic until the end of this century (including bottom water warming and freshening in particular in the Northern Basins) promote methane production, modeling of these scenarios for selected locations suggest that the trend will not lead to enhanced evasion of free gas
• The shallow upper gas front leads to the potential for gas liberation from areas with high sediment accumulation rates of Littorina mud deposits upon mechanical disturbance or displacement of the sediments. However, due to the limited connectivity of the pore spaces, such effects will be limited to the very near surrounding of the area of perturbation.